Science / Biology Glossary

Dominance Hierarchy: A social structure among a group of animals in which one is dominant and the others have subordinate nonbreeding positions.

Dominant: Refers to an allele of a gene that is always expressed in heterozygotes.

Double Fertilization: A characteristic of angiosperms in which a pollen tube carries two sperm cells to the female gametophyte in the ovule. One sperm cell fuses with the egg cell and gives rise to a diploid embr . . . View Full Definition

Duodenum: The upper part of the small intestine.

Duplication: An extra copy of a chromosome segment without altering the number of chromosomes.

Dystrophin: Protein making up only 0.002% of all protein in skeletal muscle but which appears vital for proper functioning of the muscle. Sufferers of muscular dystrophy appear to lack dystrophin.

Eccrine Glands: Sweat glands that are linked to the sympathetic nervous system and are widely distributed over the body surface.

Ecological Niche: The role an organism occupies and the function it performs in an ecosystem; closely associated with feeding.

Ecological Time: A timescale that focuses on community events that occur on the order of tens to hundreds of years.

Ecology: The study of how organisms interact with each other and their physical environment.

Ecosystem: The community living in an area and its physical environment.

Ecotones: Well-defined boundaries typical of closed communities.

Ecotype: A subdivision of a species; a stage in the formation of a species such that reproductive isolation has occurred.

Ectoderm: The outer layer of cells in embryonic development; gives rise to the skin, brain, and nervous system. Also, the outermost tissue layer in fiatworms.

Ectotherms: Animals with a variable body temperature that is determined by the environment. Examples: fish, frogs, and reptiles.

Effector: In a closed system, the element that initiates an action in response to a signal from a sensor. In human systems, a muscle or gland often serves as an effector.

Ejaculatory Duct: In males, a short duct that connects the vas deferens from each testis to the urethra.

Electron: A subatomic particle with a negative charge. Electrons circle the atoms nucleus in regions of space known as orbitals.

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